Hormone levels not doubling

I was having some light spotting so my doctor ordered a 48 hr hCG test and an ultrasound. They estimated me at 6 weeks 4 days based on my last period, but the ultrasound showed only 5 weeks 4 days (my cycle is really irregular and I know precisely when I conceived, which would be about right). But the blood tests came back with around 6000 and rising to 6800, not close to doubling. I have another us scheduled for next week, but I'm terrified this isn't viable. Any experiences or advice?